1. Field of the Invention
The present invention relates generally to LC filters and, more particularly, to an LC filter used as a noise filter or the like.
2. Description of the Related Art
As an example of conventional LC filters, an LC filter 51 is shown in FIG. 9. This filter 51 is formed of a plurality of insulating sheets 52, and more specifically, insulating sheets 52 on which coil conductors 53, 54, 55, 56, 57 and 58 are respectively disposed, a central insulating sheet 52 on which a central capacitor electrode 60 is provided, and insulating sheets 52 used as external layers. Capacitor electrodes 59 and 61 are formed on the insulating sheets 52 adjacent to the central insulating sheet 52, with the capacitor electrodes 59 and 61 being electrically connected to the central portions of the respective coil conductors 55 and 56 on the same insulating sheet 52. The coil conductors 53 through 55 in one half of the filter 51 are electrically connected in series to each other through via-holes 62a, 62b and 62c in the insulating sheets 52 to form a first coil 63. Moreover, the coil conductors 56 through 58 in the other half of the filter 51 are electrically connected in series to each other through via-holes 62d, 62e and 62f to form a second coil 64. The central capacitor electrode 60 opposingly faces each of the capacitor electrodes 59 and 61, and a capacitor can be formed by these electrodes 59 through 61.
The above-described sheets 52 are laminated and then integrally sintered to form a laminated block. Subsequently, as illustrated in FIG. 10, input/output external electrodes 74 and 73 are respectively formed on the left and right sides of the laminated block, while ground external electrodes 75 and 76 are respectively disposed on the proximal side and the distal side of the laminated block. The first input/output external electrode 73 is electrically connected to one end of the first coil 63, while the second input/output external electrode 74 is electrically coupled to one end of the second coil 64. Further, the ground external electrodes 75 and 76 are each respectively electrically connected to one end of the central capacitor electrode 60.
In the LC filter 51 of this conventional type, the external electrodes 73 through 76 are located parallel to the axis of the coils 63 and 64, and the sheets 52 are laminated along the external electrodes 73 through 76.
Another example of conventional LC filters, such as the one disclosed in Japanese Unexamined Patent Publication No. 7-283031, is also known. That is, a plurality of insulating sheets on which coil conductors are disposed are laminated, and the coil conductors are connected in series to each other through via-holes to form a laminated block. Input/output external electrodes and ground external electrodes are then formed on the laminated block in directions parallel to the axis of the coils.
Referring back to the LC filter 51 shown in FIGS. 9 and 10, the following problems are encountered. When the distances between the input/output external electrode 73 and the coil 63 and between the electrode 74 and the coil 64 are increased, stray capacitances generated therebetween can be inhibited. If, however, the LC filter 51 is comparatively small (for example, a length of 1.6 mm and a width of 0.8 mm or smaller), the input/output external electrodes 73 and 74 are brought closer to the coils 63 and 64, respectively, as illustrated in FIG. 10, thereby increasing the stray capacitances Cs therebetween.
Further, the known LC filter is constructed such that the ground external electrodes 75 and 76 are disposed to opposingly face the coils 63 and 64 over the entire length of the coils 63 and 64, i.e., from the input terminal of the coil 64 to the output terminal of the coil 63. This increases the stray capacitances Cs. Additionally, the stray capacitances Cs are increased in proportion to the number of turns of the coils 63 and 64.
By application of this laminated LC filter 51 to a noise filter, noise entering one input/output external electrode 74 circumventively travels to the other input/output external electrode 73 via the stray capacitances Cs, as shown in FIG. 11. This deteriorates the attenuation characteristics of the filter 51 in a high frequency range, thereby failing to sufficiently exhibit performance as a noise filter.
Further, since the direction of magnetic flux .o slashed. generated in the coils 63 and 64 is perpendicular to the mounting surface 65 of the LC filter 51, an eddy current is disadvantageously produced due to this magnetic flux .o slashed. in a conductor pattern having a large area, such as a ground, on a printed wiring board 80. Accordingly, the magnetic flux .o slashed. is weakened due to the eddy current loss and further lowers its self inductance and Q-factor.